Question

(a) Distinguish between the saturated hydrocarbon ethane and the unsaturated hydrocarbon ethene by drawing their structural formulae.
(b) Addition reaction and substitution reactions are types of organic reactions. Which type of reaction is shown by:
(i) ethane
(ii) ethene?
(c) (i) Write the equation for the complete combustion of ethane.
(ii) Using appropriate catalysts, ethane can be oxidized to an alcohol, an aldehyde and an acid. Name the alcohol, aldehyde and acid formed when ethane is oxidized.
(d) (i) Why is pure acetic acid known as glacial acetic acid?
(ii) What type of compound is formed by the reaction between acetic acid and an alcohol?

Answer 1

(a)

Ethane	Ethene
Structural formula	Structural formula
All valencies of carbon are satisfied by single covalent bonds.	All valencies of carbon are satisfied by double covalent bonds.
Its general formula is CnH2n+2, where n = 2	Its general formula is CnH2n, where n = 2

(b)
(i) Ethane will undergo substitution reaction since it is a saturated hydrocarbon.

(ii) Ethene is an unsaturated hydrocarbon and thus it undergoes addition reaction.

(c)
(i) The complete combustion of ethane is given by the equation:
​ ​
​ ​$2{\mathrm{C}}_2{\mathrm{H}}_6+7{\mathrm{O}}_2\to 4{\mathrm{CO}}_2+6{\mathrm{H}}_2\mathrm{O}+\mathrm{Heat}$

(ii) The alcohol, aldehyde and acid formed when ethane is oxidised are ethanol, acetaldehyde and acetic acid, respectively.

$\underset{\mathrm{Ethane}}{{\mathrm{C}}_2{\mathrm{H}}_6}\stackrel{\left[\mathrm{O}\right]}{\to }\underset{\mathrm{Ethanol}}{{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}}\stackrel{\left[\mathrm{O}\right]}{\to }\underset{\mathrm{Acetaldehyde}}{{\mathrm{CH}}_3\mathrm{CHO}}\stackrel{\left[\mathrm{O}\right]}{\to }\underset{\mathrm{Acetic}\mathrm{acid}}{{\mathrm{CH}}_3\mathrm{COOH}}$

(d)
(i) Pure acetic acid forms a dense white coloured solid on cooling, which looks like glacial ice. Due to this, pure acetic acid is called glacial acetic acid.

(ii) An acetate or an ester is formed by the reaction between acetic acid and an alcohol. For example, acetic acid reacts with ethanol in presence of concentrated sulphuric acid to form ethyl ethanoate (ethyl acetate) as shown below:

$\underset{\mathrm{Ethanol}}{{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH}}+\underset{\mathrm{Acetic}\mathrm{acid}}{{\mathrm{CH}}_3\mathrm{COOH}}\underset{\mathrm{\Delta }}{\overset{\mathrm{conc}.{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }}\underset{\mathrm{Ethyl}\mathrm{ethanoate}}{{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_5}+{\mathrm{H}}_2\mathrm{O}$